A migration processor extends the \Drupal\migrate\ProcessPluginBase class. It expects to implement the transform() function and return the result based on the configuration provided. For example:

<?php 
 
namespace Drupal\migrate\Plugin\migrate\process; 
 
use Drupal\migrate\MigrateException; 
use Drupal\migrate\MigrateExecutableInterface; 
use Drupal\migrate\ProcessPluginBase; 
use Drupal\migrate\Row; 
 
/** 
 * Concatenates the strings in the current value. 
 * 
 * @MigrateProcessPlugin( 
 *   id = "concat", 
 *   handle_multiples = TRUE 
 * ) 
 */ 
class Concat extends ProcessPluginBase { 
 
  /** 
   * {@inheritdoc} 
   * 
   * Concatenates the strings in the current value. 
   */ 
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) { 
    if (is_array($value)) { 
      $delimiter = isset($this->configuration['delimiter']) ? $this->configuration['delimiter'] : ''; 
      return implode($delimiter, $value); 
    } 
    else { 
      throw new MigrateException(sprintf('%s is not an array', var_export($value, TRUE))); 
    } 
  } 
} 

The transform() function receives the value from the migration source, a \Drupal\migrate\MigrateExecutableInterface that defines how the migration is being triggered, the migration source row, and the property on the destination entity. Processors implement the \Drupal\migrate\Plugin\MigrateProcessInterface, which specifies the following functions:

• transform(): It returns the modified value
• multiple(): It returns whether the value is treated as a scalar value or an array; the implementation in the ProcessPluginBase class returns FALSE, which would be overridden in circumstances where the processor returns an array